Post processing bloom artefacts

I've finally figured how to add post processing to a camera, but noticed that when you increase the blurH, blurV to a certain size, it creates a lot of artefacts.. looks like tiling.. How do I avoid this?

It's just a really low quality bloom shader. You can take a look at the glsl file in the repo if you want to see. The gaussian blur kernel is very small. goodbye_cmake has a higher quality one in by default.